Unitized Regenerative Fuel Cells (URFCs) are being developed by several fuel cell manufacturers. The applications for this technology are the same as for Regenerative Fuel Cell (RFC) systems. Specific NASA applications include high altitude airships, lunar or Mars-based outposts, and other secondary battery applications where the discharge period is 1 to 2 hours long or longer.
The URFCs developed to date are all based on the Proton Exchange Membrane (PEM) technology. The key advantage of the URFC over other RFC systems is that the URFC does both the process of electrolysis of water as well as the process of recombining of the hydrogen and oxygen gas byproducts to produce electricity. Because of this advantage, a one cell stack of a URFC system replaces the one electrolysis cell stack and one fuel cell stack of the prior art RFC systems. This reduction in fuel cell stacks saves a substantial amount of weight since the cell stacks are the major components of a RFC system. Besides saving the weight of one cell stack, the plumbing, wiring and ancillary equipment for one cell stack is also eliminated.
The operation of the URFC system is also simpler. A RFC requires that when the fuel cell stack is active, the electrolysis cell stack must be kept warm to avoid freezing water lines and transient warm-up periods. Likewise, as the electrolysis cell stack is active, the fuel cell stack must be kept warm to avoid freezing water lines, excessive condensation, and transient warm-up periods. Maintaining cell stacks in standby conditions complicates the overall system design, resulting in greater mass, volume, and parasitic power.
Early efforts to develop a regenerative cell resulted in cells with poor performance or cells not easily reversed in their operation. Dedicated fuel cells or electrolysis cells often have the reactants circulated through the cell stack. Usually the circulating reactants function to remove the byproducts of the fuel cell reaction (product water during fuel cell operation, and product gases during electrolysis cell operation). Sometimes the reactants are also circulated for cooling of the cell stack during its operation. For a URFC to act without circulation pumps requires that the reactants not be circulated through the cell stack, but instead, be “dead-ended” into the cell stack.
As an energy storage system, the URFC system “charges” and “discharges” like a rechargeable battery. While charging, the URFC operates the electrolysis process, which splits water into hydrogen and oxygen. While discharging, the URFC operates the fuel cell process, which combines hydrogen and oxygen and produces electricity.
The gases produced during electrolysis are expelled from the cell stack by the production of still more gas inside the cell stack. The continued production of gases by the cell stack pushes the gases into the reactant storage tanks, gradually “pumping” the gases to higher and higher pressure where they are stored. In addition to the oxygen and hydrogen, a certain level of water vapor also accompanies these gases when they are expelled from the cell stack.
During the URFC fuel cell process, as gases are consumed inside the cell stack, more gas is delivered to the cell stack by the pressurized reactant storage tanks.
The management of reactants inside the URFC cell stack is highly influenced by both the materials and the construction inside the cell stack. Besides the development of the reversible electrodes, proper and reliable reactant management inside the cell stack is most important to achieving acceptable URFC performance. Achieving this level of reactant management inside the cell stack during both electrolysis and fuel cell operation, and the transitions between these different processes, is currently the single biggest hurdle yet to be accomplished.
In the past, a number of reversible or regenerative fuel cells designs were known.
For example, U.S. Pat. No. 3,975,913 to Erickson discloses a closed-cycle gas generator in which one chemical, such as water, is reacted with a metal, such as molten aluminum, to produce hydrogen gas which, along with O2 from a separate storage tank, is conveyed to a fuel cell. Waste heat from the gas generator drives a closed-cycle heat engine.
U.S. Pat. No. 4,490,445 to Hsu discloses a reversible “solid oxide electrochemical energy converter” having a counter-flow heat exchanger that disposes of waste heat by directing it to heating the incoming fuel gases.
U.S. Pat. No. 5,338,622 to Hsu, et al discloses a fuel cell system that specifically addresses the issue of waste heat management by means of a counterflow heat exchanger assembly.
U.S. Pat. No. 5,401,589 to Palmer, et al discloses a fuel cell with a reformer, wherein the reformer receives waste heat from the fuel cell. Waste heat is also conveyed to a space heating system and also to a ‘bottoming cycle’ engine and, if at too low a temperature, is discharged to the atmosphere.
U.S. Pat. No. 5,506,066 ('066) to Sprouse discloses an ‘ultra passive,’ variable pressure RFC having a single H2 storage tank “that encloses a plurality of smaller gaseous O2 storage tubes.” No pumping elements are used. A heating/cooling coil inside the H2 tank prevents icing or overheating. The source or sink of the heat for said coil is not specified.
U.S. Pat. No. 5,510,202 ('202) to McCoy discloses a variable pressure fuel cell. McCoy uses prior art images similar to those of in the '066 patent, suggesting a possible similarity of the '202 patent's waste heat management system to that of the '066 patent.
U.S. Pat. No. 5,678,410 to Fujita et al discloses a fuel cell system for use with cars. For example. “The combined system preferably includes a heat storage tank disposed in a conduit of a heating medium . . . .” A heat transfer medium, “such as water,” is used to convey heat to various heat exchangers including one that moves heat to or from a metal-hydride storage tank.
U.S. Pat. No. 5,885,727 to Kawatsu discloses a fuel cell arrangement in which waste heat is conveyed, by way of apparently integral cooling water tubes, to reaction tanks 30 and 50 wherein, respectively, oxygen and hydrogen are generated for use in the fuel cell.
In addition to the above US patents, a PCT patent US 2003/001721 A1 to Wattelet et al discloses a mobile and compact fuel cell system having “an integrated heat exchanger unit” that combines the fuel cell cooling system and a cathode exhaust gas condenser, both being cooled, in parallel, by a shared cooling air stream. A cooling tube (callout number 38) appears integrated with the fuel cell.